A knock detecting apparatus, as shown in JP-6-60621B, has a knock sensor disposed on a cylinder block of an engine. The knock sensor detects a knock vibration and outputs a knock signal. Knock frequency components corresponding to the engine knock are derived from the knock signal by a band-pass filter. A peak value of the knock frequency components is detected by a peak-hold circuit, and then the peak value is compared with a knock determining level to determine a presence or absence of the engine knock. Alternately, the knock frequency components in a predetermined range are integrated, and the integrated value is compared with the knock determining level.
However, the frequency band generated by the engine knock is not always the same. The frequency band having a high vibration intensity is different every engine knock. Thus, when the only one of frequency band is determined and when the determined frequency band has a lower vibration intensity than the other frequency band, the engine knock can not be detected.
When an electric and/or mechanical noise occurs in the frequency band near the knock frequency components and when the knock determination is performed only from the peak value or integrated value of a knock frequency components, then an electric and/or mechanical noise may be superposed on the derived knock frequency components, and then the noise is determined mistakenly as the engine knock.
On the other hand, JP-2001-227400A shows that the knock determination is executed on the basis of the relation between the peak value in the knock frequency components and the vibration period. However, in such a knock determination, when multiple noises are superposed on the knock frequency components, it is hardly distinguish the engine knock from the noises in order to detect the engine knock correctly.